CNET published a very interesting review of all the available alternatives products (either free or paid) to Microsoft Office. The product reviewed are:

• Microsoft Office 2007
• Corel WordPerfect X3
• Sun StarOffice 8
• OpenOffice 2
• iWork ‘08
• IBM Lotus Symphony
• ThinkFree Office 3
• Google Docs & Spreadsheets
• Zoho Office

The table available here summarize the CNET rating, price and features (included the supported Operating Systems). The 2 products gettings the best ratings after Microsoft are Google Docs & Spreadsheets  and  ThinkFree Office 3.